In chemical process operations, there are various instances in which it is desired to mix fluids at different temperatures, typically a vapor at the higher temperature with a liquid at the lower temperature, whereby the liquid is vaporized by contact with the hot vapor. Particularly where the liquid, when in liquid form, is highly corrosive to the surrounding environment, such as to vessel walls, valves, and the like, it is desirable to vaporize the liquid as quickly as possible and to reduce or minimize condensation that could cause corrosion of the equipment. One important application of these principles is in connection with the in-situ stabilization and/or regeneration of catalyst used in dehydrogenation processes.
For example, as described in U.S. Pat. Nos. 5,461,179; 5,686,369; 5,695,724 and 5,739,071, adding an alkali metal to the feed of a catalytic dehydrogenation reaction system can regenerate and/or stabilize the activity of the catalyst. A prime application of the basic process concept in the above patents is for the dehydrogenation of ethylbenzene to styrene in the presence of steam over a potassium promoted iron oxide catalyst. In this case, potassium is added to a reactor feed stream to improve both the conversion of ethylbenzene and the selectivity to styrene, as is described in the examples given in these patents. This potassium can be introduced either as potassium metal or a potassium compound such as potassium hydroxide (KOH). If metal is used, it can be introduced into the reactor feed as a solid, liquid (melting point 64° C.), or a vapor (normal boiling point 774° C.) and, when the potassium metal contacts the steam in the reactor feed, it converts to potassium hydroxide. If potassium hydroxide is used, it can be introduced as a solid, liquid (melting point 406° C.), or as an aqueous solution. No matter what the source of potassium, however, the potassium should be vaporized completely and mixed thoroughly with the reactor feed prior to the feed reaching the catalyst in the reactor for improved or optimum results.
Potassium metal is highly reactive; and thus, for safety reasons, potassium hydroxide will in many cases be preferred over potassium metal as the source of potassium in a commercial catalyst stabilization operation Compared with using solid or melted potassium hydroxide, aqueous potassium hydroxide solutions will in many cases be preferred because of the ease of handling an aqueous liquid at ambient temperatures.
One difficulty with using potassium hydroxide, however, is that it can be very corrosive, especially at elevated temperatures, either as an aqueous solution or as melted potassium hydroxide. Once the potassium hydroxide is fully vaporized and stays in the vapor phase, corrosion is typically much less of a problem If an aqueous solution is injected directly into the main process piping, then corrosion of the main process piping and equipment is possible. The KOH solution will contact the vessel walls because the short distances between solution injection point and the pipe walls typically will not allow adequate vaporization time before the potassium hydroxide, as solution, solid or liquid salt, reaches the walls. Furthermore, if the injection system is directly part of the main process piping, then the dehydrogenation process must be shut down if maintenance is required for the injection nozzle assembly.
To avoid damage and downtime for the dehydrogenation process unit, which is often a large and expensive process unit, we have found in accordance with this invention that it is advantageous to take at least a part of the steam being fed to the dehydrogenation process, vaporize the potassium hydroxide into it, and then mix this potassium-rich steam with the rest of the reactor feed. To vaporize the KOH solution, the potassium hydroxide solution can be sprayed into the steam portion inside a small, dedicated “mixing vessel,” which can be shut down for periodic maintenance without shutting down the entire dehydrogenation process. If the mixing vessel and spray nozzle assembly are designed properly, the KOH solution droplets can be vaporized before the droplets reach the walls of the vessel or the vessel outlet pipe, thereby, at least in theory, reducing or minimizing corrosion caused by unvaporized solution However, in our experience of utilizing such systems, we have found that this approach is insufficient by itself to avoid significant corrosion of the mixing equipment.
Part of the problem of using potassium hydroxide in such applications is that its vapor pressure is low even at the high dehydrogenation reaction temperatures. At 598° C., which is the reactor inlet temperature of Example 1 in previously mentioned U.S. Pat. No. 5,461,179, for example, the vapor pressure of potassium hydroxide is only 10 pascals. If the total pressure is 100 kilopascals at this temperature, then the concentration of potassium hydroxide in the vapor phase cannot exceed 100 parts per million on a molar basis even at this high temperature. At 514° C., the saturation concentration would be only 10 parts per million on a molar basis. Thus, the potassium must be diluted by relatively large amounts of high-temperature steam to get the potassium totally into the vapor phase.
Even if the average conditions of the steam fed to the mixing vessel are adequate to vaporize the aqueous potassium hydroxide solution, however, we have found that the potassium hydroxide vapor can re-condense if the interior surface temperature of the mixing vessel walls is below the dew point of potassium hydroxide. Such condensation on the mixing vessel walls can cause severe corrosion because of the highly corrosive nature of liquid potassium hydroxide at the high temperatures needed for vaporization.
Although it might be expected that the temperature of the walls in such a mixing vessel would be nearly the same as that of the vapor passing through the interior, we have found that the wall temperature can be surprisingly colder than the average steam temperature. We attribute this to the following technical factors: 1) heat loss to the environment through the walls, even with a thick layer of external insulation, can be substantial; 2) heat loss is almost always even higher at vessel nozzles and supports and 3) heat transfer from the steam to the walls can be poor because of the low steam velocity resulting from the vessel volume and geometry needed for complete vaporization of the KOH solution without impinging droplets on the walls. We have determined that the differential temperature between the vapor in the interior of the mixing vessel and the wall of the vessel can be in the range of 50 to 100° C. for conventionally-designed vessels insulated according to industrial standards for energy conservation.
In practice, parts of the mixing vessel walls can be significantly colder than this at vessel support points and at vessel nozzles where heat loss can be greater and/or heat transfer from the process steam can be slower. For example, the temperature of a manway lid in such a vessel can be substantially colder than the walls of the main part of the vessel because there is no flow past the manway lid due to the fact that it is in a cul-de-sac. In contrast, the wall temperatures of regular cylindrical pipes usually will be close to the temperature of the contained fluid flow because the economic sizing of pipes typically results in significant fluid velocities, which result in good heat transfer and thus low temperature differences between the contained flow and the pipe wall.
We explored a number of possible approaches to try to solve this problem of condensation due to “cold” vessel walls using commercially available equipment and by adapting conventional technologies. As discussed below, none of these approaches proved to be entirely satisfactory.
First, we considered increasing the steam flow, which decreases the dew-point temperature of the potassium hydroxide vapor by diluting it and decreases the temperature drop somewhat of the steam through the system due to heat loss if the heat loss does not increase proportionally more than the increase in the steam flow. However, we determined that increasing the steam flow results in a proportionally larger mixing vessel so as to maintain the vessel residence time needed for droplet vaporization; heating costs for the overall process increase because heat losses are increased with the larger mixing vessel and larger diameters of the associated piping; and, even beyond the cost of making up for additional heat loss, the cost of heating for the overall process is larger because the efficiency of heating this small steam flow for the mixing vessel typically will be lower than for the dehydrogenation process. Thus, increasing the steam flow enough to make a significant difference in corrosion protection substantially increases both the capital and operating costs.
A second approach we considered was that perhaps the mixing vessel could be insulated more effectively to lower the heat loss and, thus, increase the vessel wall temperature. However, we determined that increasing insulation thickness results in diminishing returns; and, at high temperatures, heat loss still can be substantial even with thick layers of insulation. Also, heavily insulated nozzles and manways on vessels at high temperatures can be problematic because if the nozzle flange bolts are under the insulation and kept very hot (above about 565° C. for stainless steels) they become loose because of high temperature “creep” whereby the bolt metal permanently stretches because of the combination of tension imposed from tightening and temperature. Once they stretch, the bolts do not put sufficient force on the flanges to the vessel sealed. Therefore, there is an incentive to not heavily insulate the flanges, but this practice leads to high, localized heat losses and, thus, cold spots on the mixing vessel wall where condensation can occur.
A third approach we considered was to add electric heaters or electric tracing to the outside of the mixing vessel underneath the insulation. At temperatures above about 550° C., however, this approach leads to high cost with the technologies available. Furthermore, because heat loss is not uniform from the mixing vessel because of nozzles, vessel supports and insulation imperfections, control of the electric heaters would be complicated. The metal temperatures must be high enough at all points exposed to the potassium hydroxide vapor so as to avoid condensation, but care must be taken to avoid overheating the mixing vessel walls, which can lead to unacceptably low metal strength. Also, as discussed above, the bolts on the nozzle flanges on very hot equipment preferably should not be as hot as the vessel contents to prevent leakage due to high temperature creep. This approach therefore would result in either the nozzles being “cold” spots for condensation and corrosion or, alternatively, locations for increased risk of leakage, depending on whether or not the electric heaters apply heat in the area of the nozzle flanges.
A fourth approach we considered was to install an external jacket on the mixing vessel such that a hot utility stream could be passed through the jacket to warm the vessel. However, such a jacket would need to be designed for the high temperatures and the pressure of the utility fluid. It would be difficult or even impractical to adequately jacket nozzles including manways, even if this is considered to be desirable given the potential sealing problems at high bolt temperatures. Furthermore, for such systems as described above, the temperatures required exceed the highest condensing temperature for steam and the maximum operating temperatures for commercially available organic heat transfer fluids. Thus, the heat transfer fluid in an external jacket system most likely would need to be a molten salt or liquid metal, which are difficult to use, and this results in very high operating and capital costs.
A fifth approach we considered was that the steam supply temperature to the mixing vessel could be increased, which increases all of the mixing vessel temperatures and increases the difference between the mixture temperature and the dew point of the potassium hydroxide. However, there are metallurgical limits to how high the temperature can be. For temperatures up to 815° C. (1500° F.), various 300-series stainless steels can be used to construct pressure-containing vessels and pipes. At higher temperatures, however, more expensive metals must be used, and maintenance costs increase. In general, though, increasing the operating temperature up close to the limit of metallurgy is a reasonable approach to reducing or minimizing the necessary flow rate of the dilution steam.
A sixth approach we considered was that the mixing vessel wall metal could be upgraded to an alloy able to withstand, if possible, the corrosion caused by condensing potassium hydroxide. Because of the highly aggressive nature of potassium hydroxide at these high temperatures, however, the metal costs can become prohibitively expensive, which results in a large increase in capital cost for the mixing vessel. Furthermore, if the potassium hydroxide is allowed to condense, it will accumulate in the vessel, require periodic removal, and will not be fed to the reaction system as desired.
These and other deficiencies in or limitations of the prior art and the varied considered adaptations of more conventional technologies to try to address the condensation problem are overcome in whole or in part by the improved methods for condensation reduction of this invention and the related mixing vessel design